Nitrogen protoxide is a harmful greenhouse gas that is much more powerful than carbon dioxide; moreover, in the stratosphere it takes part in the reactions that lead to the destruction of the ozone layer.
The main industrial sources for the formation of nitrogen protoxide are plants for the production of nitric acid and adipic acid (monomer used in the preparation of nylon 6,6 and 6,12).
Nitrogen protoxide is present in the emissions of adipic acid plants in considerable amounts: a typical composition comprises, in percentage by volume: 30% N2O, 2% CO2, 2.5% H2O, 8-12% O2, 50-150 ppm NOx, remainder N2.
The emissions of nitric acid plants generally contain 300-1700 ppm of N2O, 100-2000 ppm of NOx, 1-4% O2, and nitrogen as the remainder.
N2O emission from nitric acid and adipic acid plants is expected to grow by approximately 16% in the period 2005-2020.
Several catalysts used for the decomposition of N2O are known. The main ones are constituted by noble metals supported on metallic oxides of different kinds, zeolites substituted with transition metal ions or on which metal oxides and anionic clays are supported, such as for example hydrotalcites constituted by mixed hydroxides with stratified structure, in which anions of different kinds, exchangeable or not, and water molecules are inserted between two layers.
All these catalysts suffer the drawback that they are not thermally stable: the noble metals supported on metal oxides because at high temperatures the particles of the metal tend to sinter, with consequent deactivation of the catalyst; clays and zeolites because their structure tends to collapse and thus lose the initial catalytic properties.
Catalysts are known (U.S. Pat. No. 5,705,136) which are constituted by oxides such as MnO, CuO, NiO and CoO supported on MgO, CaO, ZnO, TiO2, Al2O3—ZnO, Al2O3—TiO2 and the like. Preferably, the catalysts contain CoO supported on MgO.
N2O conversions are high.
US 2004/0179986 A1 mentions a catalyst of N2O decomposition that is active at temperatures between 250 and 450° C. but inactive at higher temperatures and is constituted by a mixture, in equal parts by weight, of a Co3O4 spinel and an anion defect perovskite of formula La1-xCuxCoO3-d (x≦0.5).
The US application stresses that the perovskite-like catalysts undergo deactivation when used at high temperatures (700° C.-1000° C.) if supported on alumina caused by deactivation reactions of alumina with the active catalyst phase.
Structures of the type of hydrotalcite, such as for example Cu3Mg5Al2(OH)20CO33H2O, Mn3Mg5Al2(OH)20CO3H2O, are also usable.